1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a preliminary ejecting method, and more particularly to a preliminary ejecting operation for preventing a defective ejection from a print head.
2. Description of the Related Art
If no ink is ejected from a print head of an ink jet printing apparatus such as an ink jet printer for a certain time or longer, then the viscosity of ink in nozzles increases to cause a defective ejection. In particular, the recent trend to eject finer ink droplets leads to a relative increase in the effect of the viscosity on ink ejection, as well as a reduction of ejection energy. Thus, the defective ejection caused by an increase in the viscosity of ink tends to be more serious.
Ejection recovering processes are known which prevent such a defective ejection. The ejection recovering process is executed at predetermined timings or when the temperature, printing duty, and the like of the printing apparatus meet predetermined conditions.
A well-known ejection recovering process is so-called a suction recovery process that sucks ink through the nozzles of the print head to forcibly discharge and remove ink having an increased viscosity (high viscosity). Another well-known ejection recovering process is a pressurization recovery process that pressurizes inside the print head to discharge ink through the nozzles in contrast to the suction recovery process. Furthermore, a more simple known ejection recovering process is so-called a preliminary ejecting operation that discharges ink having the increased viscosity by executing a predetermined number of ejections to a predetermined location of the printing apparatus, the ejections eventually having no contribution with the printing. Such a preliminary ejecting operation is relatively frequently executed because it is simple and does not require much time.
In a serial-type printing apparatus that executes printing by scanning a print head over a print sheet, the print head is generally moved to a predetermined location outside a printing area, where the preliminary ejecting operation is performed. On the other hand, so-called a full-line printing apparatus is known which executes printing while transporting a print sheet relative to a print head having nozzles arranged within a range corresponding to the width of the print sheet. In the case that a plurality of print sheets are continuously transported for printing by the full-line printing apparatus, a preliminary ejecting operation is performed on an area different from the print sheet, for example, on a transport belt. In these conventional cases, several tens of ejections (several tens of droplets) are executed to appropriately remove ink having the increased viscosity during the preliminary ejecting operation.
The preliminary ejecting operation is often performed each time a predetermined amount of printing is completed. For the serial printing apparatus, the preliminary ejecting operation is performed, for example, at the intervals of a predetermined number of scanning operations or after each printing process for one page. In this case, the print head is moved to an ink receiver provided at a predetermined location where a preliminary ejecting operation is performed. On the other hand, for the full-line printing apparatus, a preliminary ejecting operation is performed on the transport belt as described above after a printing process for one page has been completed and before the next page is printed.
Such a conventional preliminary ejecting operation enables defective ejections to be prevented regardless of the degree of an increase in the ink viscosity, which varies in the nozzles. That is, ink is not ejected through some of the nozzles according to print data, and the ink in these nozzles undergoes a significant increase in viscosity. On the other hand, ink in nozzles continuously ejecting may not be subjected to an increase in viscosity. In spite of such a variation in the degree of the increase in viscosity among the nozzles, by performing the above preliminary ejecting operation at a predetermined timing, defective ejections can be appropriately prevented without any configuration for detecting the degree of the increase in viscosity of each nozzle.
However, in the serial printing apparatus, the print head is moved to the predetermined location before performing the above-described preliminary ejecting operation. This requires an amount of time including that required to move the print head, thereby possibly hindering the throughput of the printing apparatus from being improved. On the other hand, in the full-line printing apparatus, a relatively large amount of ink is ejected to the belt during the preliminary ejecting operation. Thus, the conventional full-line printing apparatus requires a separate cleaning mechanism to remove the relatively large amount of ink from the belt.
In order to solve the above problems, a method until now has been known which ejects ink to, for example, an area on a print medium such as a print sheet where no image is formed. However, with this method, several tens of ink droplets are ejected during the conventional preliminary ejecting operation, so that a relatively large amount of ink droplets adhere to the print medium. Accordingly, dots formed by ink droplets removed from the nozzles are easily perceived in an image formed on the same print medium, thereby possibly degrading the entire image.
It is an object of the present invention to provide an ink jet printing apparatus and a preliminary ejecting method that can solve the above-described problems of the conventional preliminary ejecting operation, that is, a decrease in throughput or the necessity of a separate cleaning mechanism.